1. Field of the Invention
The present invention relates generally to a magneto-optical disc recording apparatus and, more particularly, is directed to a position control apparatus for use with a magneto-optical disc recording apparatus for controlling the position of a magnetic field coil provided as external magnetic field generating means.
2. Description of the Prior Art
Recently, in the technical field of the disc recording apparatus, a magneto-optical recording apparatus is proposed as a system which can record a magneto-optical disc.
In the magneto-optical recording apparatus, a disc in which a perpendicular magnetic recording medium is utilized as a record medium has an external magnetic field applied to it by external magnetic field generating means such as a magnetic field coil or the like. Under this condition, a laser beam is irradiated on the above-described disc to locally raise its temperature, whereby a coercive force in the portion irradiated by the laser beam is decreased to invert a magnetization by the external magnetic field, thereby a signal being recorded or erased.
A permanent magnet or a magnetic field coil may be utilized as external magnetic field generating means for applying an external magnetic field to the perpendicular magnetic recording medium of the disc. When a permanent magnet is employed as the external magnetic field generating means, an inverting mechanism for inverting the permanent magnet must be provided. There is a limit to the speed at which the magnetic field of the permanent magnet can be rapidly inverted by the inverting mechanism. Therefore, the laser beam is modulated on the basis of data to be recorded and the recording is made (this system is what might be called an optical modulation system).
Whereas, when a magnetic field coil is employed as the external magnetic field generating means, the inversion of the magnetic field can be rapidly performed by changing-over a current to be supplied to the magnetic field coil. Therefore, a current to be supplied to the magnetic field coil is modulated on the basis of data to be recorded and then the recording is carried out (this system is what might be called a magnetic field modulation system). In the magnetic field modulation system, a so-called overwrite is possible so that the time necessary for erasing unnecessary data on the disc is not required. Thus, the magneto-optical disc can be treated as a standard magnetic disc. Further, the above-mentioned inverting mechanism becomes unnecessary and there is then the advantage that the magneto-optical recording apparatus can be made compact in size.
Accordingly, in the magneto-optical disc recording apparatus utilizing the magnetic field coil provided, for example, as the external magnetic field generating means, in order to apply a sufficient external magnetic field to the disc so that a signal can be positively recorded or erased, the magnetic field coil provided as the external magnetic field generating means must be located close to the disc, and also the distance between the magnetic field coil and the disc must be controlled to become constant.
As the position control means or method for controlling the position of the magnetic field coil in this magneto-optical recording apparatus, the following methods are proposed:
(a) A stationary method . . . A magnetic field coil is fixedly located in advance relative to the disc with a spacing larger than a tolerance of its surface wobbling; PA0 (b) A method utilizing a tire . . . A tire is provided to rotatably contact with a surface of the disc and a block including a magnetic field coil is provided on a rotation center axis of this tire (see Japanese Laid-Open Patent Gazette No. 63-206936); and PA0 (c) A method for controlling a position of a magnetic field coil by the combination of a photo sensor and an actuator. The distance from the disc to the magnetic field coil is measured by a photo sensor, and the actuator is driven by an information of the measured distance, thereby the position of magnetic field coil is controlled.
However, the prior-art position control means for controlling the position of the magnetic field coil have the following unavoidable defects:
In the stationary method (a), the distance between the magnetic field coil and the disc must be increased in order to prevent the magnetic field coil from being brought in contact with the disc when the disc is wobbled due to the warping of the disc. Consequently, a powerful magnetic field is required in order to record (or erase) a signal, which unavoidably causes the magnetic field coil to consume large amount of electric power. Therefore, this method (a) is not suitable for miniaturizing the magneto-optical disc recording apparatus or making the same as a portable one.
In the method (b) utilizing the tire, when the surface of the disc is irregular or has roughness, the tire is directly affected by the uneven surface of the disc and is thereby wobbled considerably so that the position of the magnetic field coil becomes unstable. Further, this method (b) is easily affected by an external disturbance due to the irregular surface of the disc. Furthermore, a rotary supporting mechanism for the tire needs so many assembly parts, which provides a complicated arrangement. Therefore, this method (b) is not suitable for making the magneto-optical disc recording apparatus as a portable one.
Further, in the method (c) for controlling the position of the magnetic field coil by the combination of the photo sensor and the actuator, this method is not affected by the external disturbance due to the irregular surface of the disc. However, many assembly parts such as the photo sensor, the actuator and so on are required, and also its electrical circuit becomes complicated in arrangement. This method (c) is also not suitable for making the magneto-optical recording apparatus as a portable one.